A gas turbine engine employs various coatings. Some of these coatings comprise a metal and a non metal, or a metal and intentional porosity, deposited on a metal substrate by thermal spray methods. These coatings typically have a low metal volume fraction, typically in the range of from 0.3 to 0.5, compared to that of solid metal. The coating service performance can often be predicted on the basis of one or more properties of the coatings. The performance of the coatings often has an effect on the performance of the gas turbine engine. Thus, it is desirable to be able to determine various properties of the coatings.
For example, thermal sprayed coatings are commonly used in gas turbine engines and other types of turbomachinery. The thermal spray process is capable of depositing coatings containing a wide range of metallic, ceramic and polymeric constituents. The composition of these coatings can be adjusted to provide a combination of properties, i.e., erosion resistance and abradability, that allow the coatings to deliver superior performance as compressor outer air seals.
U.S. Pat. No. 4,783,341 to Packer et al. discloses a method for making and inspecting porous metal plasma sprayed abradable seals. Radiation transmission techniques are utilized to determine the as-sprayed density of a deposit containing metal and polymer powder particles. Based on the measuring density, a mathematical prediction is made of what the surface hardness of the sprayed deposit will be after it has been machined and then heated to remove the polymer powder particles. However, this method requires the use of radioactive material.
U.S. Pat. No. 5,202,837 to Cos et al discloses that eddy current techniques can be used to track density, shape, and grain size of material during consolidation processes. However, the relative densities monitored range from an ideal of 1.0 to a low of only 0.6. Furthermore, unlike an abradable seal having a low metal volume fraction of 0.3 to 0.5, such processes typically results in a relatively uniform microstructure with few pores due to the high relative density.